WASHINGTON — A top official with the Department of Health and Human Services told members of Congress on Thursday that the agency had lost track of nearly 1,500 migrant children it placed with sponsors in the United States, raising concerns they could end up in the hands of human traffickers or be used as laborers by people posing as relatives.

The official, Steven Wagner, the acting assistant secretary of the agency’s Administration for Children and Families, disclosed during testimony before a Senate homeland security subcommittee that the agency had learned of the missing children after placing calls to the people who took responsibility for them when they were released from government custody.

The children were taken into government care after they showed up alone at the Southwest border. Most of the children are from Honduras, El Salvador and Guatemala, and were fleeing drug cartels, gang violence and domestic abuse, government data shows.

From last October to the end of the year, officials at the agency’s Office of Refugee Resettlement tried to reach 7,635 children and their sponsors, Mr. Wagner testified. From these calls, officials learned that 6,075 children remained with their sponsors. Twenty-eight had run away, five had been removed from the United States and 52 had relocated to live with a nonsponsor.

But officials at the agency were unable to determine with certainty the whereabouts of 1,475 children, Mr. Wagner said.

The new details come as Congress is examining safeguards put in place by the Department of Health and Human Services and the Department of Homeland Security to make sure children who show up alone at the border are turned over to relatives, and not human traffickers.

Senator Rob Portman, Republican of Ohio and the chairman of the subcommittee, said he was shocked that government officials did not know the whereabouts of hundreds of migrant children released from their care.

“H.H.S. has a responsibility to better track these children so they aren’t trafficked or abused, and so they show up to their court hearings,” he said.

Two years ago the subcommittee released a report detailing how health and human services officials placed eight children with human traffickers who forced the minors to work on an egg farm in Marion, Ohio. The report found that department officials had failed to establish procedures to protect the unaccompanied minors, such as conducting sufficient background checks on potential sponsors and following up with sponsors. As a result, the children were turned over to the people who contracted them out to the egg farm.

To prevent similar episodes, the Homeland Security and Health and Human Services Departments signed a memorandum of understanding in 2016, and agreed to establish joint procedures within one year for dealing with unaccompanied migrant children.

More than a year after the new guidelines were due, the two agencies have not completed them, Mr. Portman said.

Children who show up at the border by themselves are usually apprehended by Border Patrol agents or turn themselves in to customs officers at the Department of Homeland Security. Once they are processed, they are turned over to the custody of the Department of Health and Human Services’ refugee office. The office runs more than 100 shelters around the country where it houses children and provides care until they can be turned over to a sponsor while awaiting their immigration hearings.

The sponsors are usually parents or family members already residing in the United States. The sponsors are supposed to undergo a detailed background check.

After the children have been placed with sponsors, workers at the department follow up with calls to ensure that the minors continue to live with the sponsors, are enrolled in school and are aware of their court dates.

But several immigration advocates who work with unaccompanied children said the department did little follow-up. Allison E. Herre, a lawyer with Catholic Charities of Southwestern Ohio, said she had seen sponsors who forced the children to work instead of attending school and who failed to ensure that the children attended their court proceedings.

Since 2016, health and human services officials said they have started calling sponsors to check on children 30 days after placement. But the agency said it was not legally responsible for children after they had been released from its refugee office.

Mr. Wagner said the agency was re-examining its interpretation of existing laws to make sure that migrant children were not turned over to smugglers or human traffickers.

A version of this article appears in print on , on Page A14 of the New York edition with the headline: Agency Says It Lost Track Of Nearly 1,500 Child Migrants. Order Reprints | Today’s Paper | Subscribe